Characterizing the temporal variability and emission patterns of pollution plumes in the Pearl River Delta of China

The Pearl River Delta (PRD) is a fast developing region in China that has experienced serious air pollution such as high ozone (O3) and poor visibility. In this study, we present the measurement results for ozone, carbon monoxide (CO), nitric oxide (NO), total reactive nitrogen (NOy), and sulfur dioxide (SO2) obtained at a rural site in the southern PRD during October-December 2001, which is typically a season of high pollution in this region. The aim of this study is to (1) characterize the temporal variability and emission ratios of pollution plumes from the PRD region and (2) elucidate the processes (i.e., sea-land breeze and regional emission) that have contributed to the high concentrations of pollutants over the Pearl Estuary. During the 3-month period, 22 moderately high-ozone episodes (hourly O3>80ppbv) were observed, with the highest 1-h O3mixing ratio of 142ppbv. Very high levels of primary pollutants (CO∼3000ppbv, NOy∼250ppbv and SO2∼100ppbv) were also observed at this rural site. All pollutants tended to show higher mixing ratios in the daytime, which is due to the transport of urban plumes by daytime northerly winds. Cold fronts and northeasterly monsoons were the main synoptic processes that transported regional pollution to the South China coast during autumn and winter. Ozone in the afternoon did not show a simple positive correlation with CO or NOy, as observed in many rural areas, suggesting mixing of air masses of different photochemical ages. The primary pollutants (CO, NOy, and SO2) exhibited moderately strong positive correlation (r2= 0.55-0.59), with slopes of SO2/NOy= 0.23ppbv/ppbv and CO/NOy= 9.2ppbv/ppbv. These ratios reflect the combined contributions from Hong Kong and the rest of the PRD region with characteristically different emission signatures. Analysis of individual cases showed that urban Hong Kong plumes typically contained a CO-to-NOyratio of 3.9-6ppbv/ppbv, whereas regional air masses usually contained a much higher ratio of about 20. The measured ratios are compared with the ratios calculated from the latest emission inventories. The data set from this study is valuable for studying the outflow of PRD pollution and the effects on the oxidizing capacity and radiation balance of the maritime atmosphere in subtropical Asia.